Development of the fetal membranes

The fetal membranes, sometimes called extraembryonic membranes, are tissues that form in the uterus during the first few weeks of development and develop along with the growing embryo.

These are the amnion, the yolk sac, the chorion, and the allantois, and each of these membranes starts out as a thin sheet of tissue that surrounds a fluid filled cavity.

The first membranes to form are the amnion and the yolk sac.

At the beginning of week 2, the embryo is not quite an embryo yet—it’s just a blastocyst, and the blastocyst is partially embedded into the endometrium of the mother.

The blastocyst has two parts: an outer cell mass called the trophoblast and an inner cell mass called the embryoblast.

The trophoblast is divided into the cytotrophoblast, which forms the wall of the blastocyst, and the syncytiotrophoblast, which is outside the wall and invades into the uterus.

The embryoblast is a bilaminar disc with a dorsal epiblast layer and a ventral hypoblast layer.

During day 8, a tiny space called the amniotic cavity forms between the epiblast and cytotrophoblast.

The epiblast cells migrate to form a thin membrane called the amnion that surrounds the amniotic cavity, separating it from the cytotrophoblast.

During day 9, hypoblast cells begin to migrate to form a thin membrane lining the rest of the blastocoel, forming the walls of the yolk sac.

The yolk sac fills with fluid, called vitelline fluid, which washes across the embryo, nourishing it during this early stage.

So basically, by the middle of the second week, the two-layered embryonic disc is sandwiched between the amniotic cavity and the yolk sac.

By day 10, some epiblast cells differentiate into extraembryonic mesoderm, and they settle between the amniotic cavity and the cytotrophoblast, eventually creating a thick layer of extraembryonic mesoderm tissue between the two.

Then, over time, a space starts to form within this thick layer of extraembryonic mesoderm.

The cells migrate either towards the amniotic cavity and yolk sac side or towards the cytotrophoblast side, leaving behind a space in the middle.

This space is called the extraembryonic coelom or chorionic cavity, and it continues to expand, until there’s eventually just a thin layer of extraembryonic mesoderm cells lining the amniotic cavity and the yolk sac and another thin layer lining the cytotrophoblast.

The one spot where the cavity doesn’t form is at the body stalk, which is where the embryoblast still remains attached to the wall.